Multithreading is often misunderstood, even under devs. Multithreading is primary used for parallelism and not to speed things up. For example, in games multithreading is ideal to keep your game responsive while the game is loading some resources (for the next area), the user is doing some inputs, or the AI is calculating (re)actions.
Yes you can achieve speed ups with mt, and mt is often used for speed ups, for example the rendering in suites like 3ds or Maya. But your problem must be suited to be run in a parallel way. And in most cases the speed up is far away from a linear speed up. With a perfect linear speed up you will gain potentially 300% performance with a quad-core, this seems huge. But a linear speed up is unrealistic. You have to organize (Mutex, MVar, synchronize, STM) the different processes or threads on their meeting-points, and that results into a slow down. It's utopian that a whole game problem will gain a 300% speed up, even +100% is far away from reality. In most cases you will solve specific sub-problems with mt or, and that is the most common way, you decoupling sub-systems from each other to be run parallel on their own processing unit.
MT is often a trade-off. MT will make your project much more complex. More complexity will make your project more error-prone and will slow down the whole project progress. Your code-base is more fragile and "uglified". Whats the benefit? More responsiveness, that's fine!. 10%-30% "speed up", maybe not worth it.
I highly guess the EVE Online client makes use of parallelism, but not in that way the gamer expect. The gamer takes a look at the task manager and complains about the single cpu usage. But in which situation? Maybe this situation is not really parallelizable. For example: Pumping Data to the graphic card is not well parallelizable, sometime not even possible. Let me speculate: EVE Online parallelize the client view, the network and resource loading. In the huge fleet fight, when everything is loaded on the client-side, the bottleneck will be the rendering. And when the cpu part of the rendering isn't well parallelizable, the cpu usage is reduced to the only rendering core.